1. Technical Field
The present disclosure relates generally to the treatment of respiratory conditions with continuous positive airway pressure (CPAP) devices, and more particularly, to sleep detection for controlling CPAP therapy.
2. Related Art
Sleep apnea is a serious medical condition in which patient breathing during sleep pauses abnormally, or is abnormally low. Apnea is categorized as obstructive, central, and combined obstructive and central, though the obstructive sleep apnea (OSA) is the most common. The patient's upper airway repeatedly narrows or collapses, causing pauses in breathing that may extend in duration up to half a minute. Although some degree of apnea is considered normal, in more severe cases, daytime sleepiness and fatigue may result as a consequence of reduced blood oxygen saturation, as well as constant interruptions to sleep cycles resulting from patients gasping for air. There have been studies linking sleep apnea to more severe long-term health issues including heart disease and depression, and recently, to cancer as well. With apnea being strongly linked to obesity, and with obesity being projected to increase, the number of patients suffering from sleep apnea is likely to increase concomitantly.
One common treatment for obstructive sleep apnea is continuous positive airway pressure (CPAP) therapy, where a positive pressure is applied to the patient to prevent its collapse as would otherwise occur during an apnea episode. By retaining the patient's airway, normal, uninterrupted breathing during sleep is ensured. In a basic implementation, CPAP therapy applies a constant pressure that is not tied to the patient's normal breathing cycle. The positive airway pressure is desired in the inspiratory phase when the pressure differences between the lungs and the nose contribute to the collapse of the intermediate airway.
Earlier patient breathing assistance devices tended to be uncomfortable to use because of the bulkiness associated with the patient interface, as well as the misapplication of pressure resulting from sub-optimal control methodologies. Various improvements have been developed to reduce discomfort during therapy, particularly at critical points along the patient's respiratory cycle. Thus, what was previously prescribed only for the more severe cases of sleep apnea in which the benefits of treatment outweighed the significant discomfort is now useful for treating a wider spectrum of sleep apnea conditions.
Notwithstanding the increased availability of CPAP devices for home use as a result of these technical improvements, ensuring patient compliance with the prescribed treatment remains a challenge. One of the primary complaints from those patients who eventually abandon CPAP therapy is the discomfort associated with the application of pressure against respiratory efforts of the patient during a waking state. The sense of asphyxiation associated with even a slight pressure and corresponding increase in work of breathing may be distracting enough to a patient such that merely falling asleep becomes difficult.
The typical operating mode of CPAP devices requires that the patient don the ventilation interface (mask) first and then turn on the power, which in turn initiates the delivery of therapeutic pressure. If the patient has become accustomed to the pressure, then no discomfort and difficulty in falling asleep is experienced. However, some patients found that this initial pressure applied during a wakeful state is too bothersome, leading to abandonment of therapy. Earlier solutions to this issue include a ramping operating mode where the initial applied pressure is reduced to a preset level with which the patient is comfortable, and gradually increasing the pressure over a period of time until the full prescription pressure level is reached. In most cases, the duration is set to about 25 minutes.
However, there are several deficiencies and patient comfort issues that remain. Although the ramping duration is adjustable, the patient may not fall asleep during the ramp time, or conversely, the patient may fall asleep before reaching full prescription pressure and so there may be an earlier onset of obstructive events. Furthermore, following an obstructive event or other reason, the patient may prematurely awaken with the CPAP device already delivering full prescription pressure. Falling back asleep may prove difficult because of the increased pressure levels and the asphyxiation sensations resulting therefrom. Accordingly, there is a need in the art for detecting the sleeping state of a patient undergoing CPAP therapy to better control the delivery of therapeutic pressure for improved comfort.